This invention relates in general to polyimide resins and, more specifically, to stable, water soluble polyimide coating compositions and methods for making them.
Prior U.S. Pat. Nos. 4,161,477, 4,183,838, 4,183,839 and 4,442,283 disclosed and claimed certain polyimide compositions which are flame resistant and useful as coatings and adhesives.
The coating and adhesive compositions described in the above-mentioned prior patents are made by first preparing a suitable bisimide by reacting an aromatic tetracarboxylic acid dianhydride with a cyclic amide or oxoimine.
The polyimide forming material is then prepared by dissolving the bisimide in an organic solvent, such as an aliphatic alcohol; then adding thereto a suitable diamine, producing a viscous fluid containing an intimate, unpolymerized mixture of N-substituted cyclic bisimide dicarboxylic acid and diamine which is capable of being converted to a high molecular weight polymer by the application of heat.
The solution diluted to a suitable viscosity with an organic solvent is coated onto a surface, dried and polymerized by heating to a temperature in the 177.degree.-316.degree. C. range for 30 minutes to 5 hours. The resulting coating is tough and highly adherent to various surfaces. It has excellent peel strength and is resistant to high temperatures, peeling and abrasion.
The prior coating material, however, was not fully satisfactory for many applications. Often the prior coating compositions required the addition of flow control or wetting agents in order to have satisfactory coating characteristics. The organic solvents used in the prior coating compositions were very flammable, requiring special shipping containers and solvent recovery facilities while drying the coating to meet air quality standards. Long term storage was sometimes not practical due to danger of viscosity changes or gellation.
Thus, there is a continuing need for improved polyimide coating compositions having a better combination of long-term storage capability, improved wetting characteristics, and/or flammability.